1. Technical Field
Exemplary aspects of the present disclosure generally relate to a transfer device and an image forming apparatus, such as a copier, a facsimile machine, a printer, or a multi-functional system including a combination thereof, and more particularly, to a transfer device that transfers a toner image borne on an image bearing member onto a recording material and an image forming apparatus including the transfer device.
2. Description of the Related Art
A known image forming apparatus such as disclosed in JP-4040611-B1 (JP-2006-39401-A) forms a charged latent image on a surface of an image bearing member such as a photosensitive drum by writing optically an image based on image information on the uniformly charged image bearing member. The latent image is developed with a developing device with toner to form a visible image, known as a toner image. Subsequently, the toner image is transferred onto a recording sheet (recording material), and is fixed thereon.
In the image forming apparatus of this kind, the toner image is formed on the photosensitive drum through a known electrophotographic process. In the known process, an n intermediate transfer belt formed into an endless loop serving also as an image bearing member contacts the photosensitive drum to form a so-called primary transfer nip therebetween. In the primary transfer nip, the toner image on the photosensitive drum is primarily transferred onto the intermediate transfer belt. A secondary transfer roller serving as a nip forming member contacts the intermediate transfer belt to form a so-called secondary transfer nip. A secondary-transfer opposing roller is disposed inside the looped intermediate transfer belt opposite the secondary transfer roller with the intermediate transfer belt interposed therebetween.
While the secondary-transfer opposed roller disposed inside the loop of the intermediate transfer belt is grounded, the secondary transfer roller disposed outside the loop is supplied with a secondary transfer bias (voltage). With this configuration, a secondary transfer electric field is formed between the secondary-transfer opposing roller and the secondary transfer roller so that the toner image moves electrostatically from the secondary-transfer opposing roller side to the secondary transfer roller side. A recording medium is fed to the secondary transfer nip in appropriate timing such that the recording medium is aligned with the toner image formed on the intermediate transfer belt. Due to the secondary transfer electric field and a nip pressure in the secondary transfer nip, the toner image on the intermediate transfer belt is secondarily transferred onto the recording medium.
In recent years, a variety of recording media such Japanese paper known as “Washi” have come on market. Such recording media have a coarse surface through embossing process. A pattern of light and dark patches according to the surface condition of the recording medium appears in an output image. Toner does not transfer well to such embossed surfaces, in particular, the recessed portions of the surface. This inadequate transfer of the toner appears as a pattern of light and dark patches in the resulting output image.
In the known image forming apparatus, the intermediate transfer belt employs an elastic intermediate transfer belt in which an elastic layer made of urethane rubber and silicone rubber is formed on the base layer of the intermediate transfer belt and fluororesin or the like is used for the surface layer thereof. Such an intermediate transfer belt allows the elastic layer thereof to deform in the belt thickness direction due to the pressure of the secondary transfer nip when transferring the toner image from the intermediate transfer belt onto the recording sheet having a rough surface. Accordingly, the surface of the intermediate transfer belt and the recording sheet, between which the toner is interposed, contact well, thereby transferring reliably the toner image onto the recording sheet.
JP-2012-128229-A also discloses an image forming apparatus including an elastic intermediate transfer belt having an elastic layer. The image forming apparatus includes a pressing mechanism which can change a pressing force of the secondary transfer roller relative to the intermediate transfer belt. The pressure of the secondary transfer nip is increased when forming an image on a recording material with high surface roughness such as fabric. The pressure of the secondary transfer nip is decreased when forming an image onto a recording sheet with low surface roughness such as a gloss resin sheet. With this configuration, for each of a wide variety of recording sheets, secondary transfer is performed at an optimum secondary transfer nip pressure to obtain satisfactory transfer efficiency.
As disclosed in JP-2012-128229-A, in a case in which the elastic intermediate transfer belt is used, it is desirable that the secondary transfer be performed at a suitable secondary transfer nip pressure depending on types of recording sheets with different surface unevenness to obtain satisfactory transferability. However, as the number of types of corresponding recording sheets is increased, a change width of the secondary transfer nip pressure is widened. Therefore, the range of the secondary transfer nip pressure changeable in the known pressing mechanism is difficult to accommodate the necessary change width of the secondary transfer nip pressure. For example, in the pressing mechanism such as in JP-2012-128229-A, the pressing force of the secondary transfer roller is changed from 50 [N] to 75 [N], but the change width is insufficient.
More specifically, in the known pressing mechanism capable of changing the secondary transfer nip pressure, generally, the pressing force of the secondary transfer roller against the intermediate transfer belt is changed by changing the compression amount and the tension of springs such as a compression spring and a tension spring. In this configuration, when the secondary transfer nip pressure is changed significantly, a significant change in the compression amount and the tension of the springs is necessary. Therefore, the springs which can change significantly the compression amount and the tension, in other words, springs (elastic members) having a wide elastically deformable range are necessary. However, manufacturing such spring members is not easy and increases the manufacturing cost. It is thus difficult to obtain the necessary change width of the secondary transfer nip pressure.
By contrast, when using a spring (elastic member) having a relatively high spring constant (modulus of elasticity), a spring having a relatively narrow, elastically deformable range may be used even when the secondary transfer nip pressure is changed significantly. However, in the spring having a high spring constant, the rate of change of the restoring force with respect to the unit compression amount or the unit tension amount is too high. Consequently, the sensitivity of the secondary transfer nip pressure with respect to the compression amount or the tension amount (elastic deformation amount) of the spring member is increased.
The secondary transfer nip pressure thus easily deviates from the target value due to the slight deviation and error of the compression amount and the tension amount of the spring so that it is difficult to reliably obtain the target secondary transfer nip pressure. The increase in the spring constant (modulus of elasticity) is thus limited. In the known configuration which requires the spring (elastic member) having a wide elastically deformable range, it is difficult to obtain the necessary change width of the secondary transfer nip pressure.
The similar difficulty may arise in an image forming apparatus in which a significant change in the transfer nip pressure is required.
In view of the above, there is demand for a transfer device capable of reliably obtaining a target transfer nip pressure using an elastic member with a relatively narrow elastic deformation range in the case of changing a transfer nip pressure, and an image forming apparatus including the transfer device.